ita a great lens for low light and will give the same angle of view as a 50mm in film equilivant, and 50mm was considered the same angle of view as the human eye. so scenes at that angle looks more realistic and natural

Sorry I do not own the any of the Sigma prime lens that you mentioned but I have done a lot of research and read a lot about those lens. Before I bought my Pentax 40mm limited, I was looking to buy the Sigma 30mm f/1.4. I know they are different in focal and speed, but I went with the limited mainly because of the size and to take, price and I wanted to take advantage of the rebates before they were gone.

From what I've read, the Sigma 30mm f/1.4 if a really great lens and is a favorte with many other photographers as well, not just Pentaxian.

I'm not much of a wide angle/normal guy, but here's my 2¢ from reading both user and tech reviews.

The Sigma is very sharp at the center, but the edge resolution is significantly lower. Apparently this was done by choice by the lens designers. I would probably go with this lens over the 31 LTD because of the huge price differential, but if price became no object, I would go with the 31 LTD without question. It's reported to be the best of the LTDs in rendering qualities -- very sharp from wide open across the frame, and it only gets better stopped down. -- very creamy bokeh, regardless of background texture. . .

If the 31 LTD is way out of your price range, then this is a viable alternative, but it's got its compromises.